Many systems for production of three-dimensional models by photohardening have been proposed. These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photohardenable composition with a laser beam according to a desired pattern and building a three-dimensional model layer by layer. In addition to various exposure techniques, several methods of forming thin liquid layers are described which allow either the coating of a platform initially or the successive coating of object layers previously exposed.
U.S. Pat. No. 4,575,330, (C. W. Hull), issued on Mar. 11, 1986, and later reexamined (certificate issued on Dec. 19, 1989), describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment, or chemical reaction, wherein successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process.
Publication "Automatic Method for Fabricating a Three-Dimensional Plastic Model with Photohardening Polymer" by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773, Nov. 1981, describes a method for automatic fabrication of a three-dimensional plastic model. The solid model is fabricated by exposing liquid photo-forming polymer, of 2 mm thickness or less, to ultraviolet rays, and stacking the cross-sectional solidified layers. Publication "Solid Object Generation" by Alan J. Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, August 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, simple three-dimensional objects from information stored in computer memory. A good review of the different methods is also given by a more recent publication, titled "A Review of 3D Solid Object Generation" by A. J. Herbert, Journal of Imaging Technology 15:186-190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photoformable composition with a laser beam according to a desired pattern. In addition to various exposure techniques, several methods of forming thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
Such methods of coating suggested thus far, however, have drawbacks in that they are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, or they do not effectively prevent damage to previously formed layers during the successive coating process. Furthermore, most previous methods omit to recognize very important parameters involved in the coating process such as, for example, the effects of having both solid and liquid regions present during the formation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photoformed layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as, for example, hydrogen bonds and substantially stronger forces such as, for example, mechanical bonds and vacuum or pressure differential forces on those thin layers and on the object being formed.
The Hull U.S. Pat. No. 4,929,402, for example describes a dipping process where a platform is lowered either one layer thickness or is dipped below the distance of one layer in a vat then brought up to within one layer thickness of the surface of the photohardenable liquid. Although theoretically most liquids will flatten out due to surface tension effects, high viscosity liquids and even low viscosity liquids take an inordinate amount of time to flatten to an acceptable degree especially if large flat areas are being imaged and if the liquid layer thickness is very thin. Regions where previous layers consist of solid walls surrounding liquid pools further compounds the flattening process of the thin liquid layer coating. In addition, motion of the platform and parts, which have cantilevered or beam (regions unsupported in the Z direction by previous layer) sections, within the liquid creates deflections in the layers contributing to a lack of accuracy in the finished object.
The Munz patent (U.S. Pat. No. 2,775,758, issued in 1956) and Scitex application describe methods by which the photohardenable liquid is introduced into the vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have all the problems of the Hull methods except that the deflections of the layers during coating is reduced.
Pertinent art relative to viscosity reducible compositions and shear-thinning compositions, including viscosity reducible photopolymer compositions, exists. However, previous compositions have been developed to solve substantially different problems than those encountered in Solid Imaging. For example, inks and paints have been formulated to be shear-thinning to prevent sagging and smearing after application. These problems of sagging and smearing are not a primary consideration in Solid Imaging processes. However, as will be seen in the disclosure to follow, the shear-thinning properties of these compositions provide significant advantages, unanticipated by the prior art, when the substantially different problems and considerations inherent in the substantially different Solid Imaging process are taken into account.
The nature of viscosity reducible compositions used in a Solid Imaging process, as taught in this disclosure, entails that the composition must at some time be of relatively high viscosity during the practice of the invention described herein. However, other pertinent art, such as Hull in U.S. Pat. Nos. 4,929,402 and 4,575,330, European Patent Applications EP 0 355 944 A2, WO 89/10254, WO 89/10255, WO 89/10249, and EP 0 354,637, teach away from the use of high viscosity compositions in the practice of Stereolithography. In addition, the DeSoto patents, such as for example U.S. Pat. Nos. 4,844,144, 4,945,032, and 4,942,001, (Murphy et al.) all teach that low viscosity compositions are necessary for the Stereolithography process. Likewise, Laid-Open-To-Public Japanese Patent Application No. Hei 1(1989)-204915, of Japan Synthetic Rubber Co. (Kobayashi et al.) teach that low viscosity compositions are required. Similarly, a Japanese patent application Kokai Patent No. 14133 from Mitsubishi (Maruya et al.) teach heating of substantial portions of the composition in a vat to lower the composition viscosity and thereby improve the speed and sensitivity of the composition in a Solid Imaging process. Similarly, Sony has filed a European Patent Application, Publication No. 0 376 571 that teaches ways to lower the viscosity of the composition used in a Solid Imaging apparatus by various methods of heating the composition. Clearly, experts in the art teach that photopolymer compositions, used in a three-dimensional layer upon layer processes, are preferably low in viscosity. This is typically true for all apparatus that employ low shear coating methods.
On the other hand, in a recent European Patent Application Publication No. 0361847 by 3-D Systems (Almquist et al.), wherein a doctor blade is utilized to form flat photopolymer composition layers, moderately higher viscosity compositions are generally recommended. However, to avoid the hydrodynamic effects involved in using these moderate viscosity compositions, mechanical fix recommendations are made, such as raising the object under manufacture above the level of the composition surface in the vat and scraping the blade over just the object to remove the composition without scraping the remaining composition in the vat, varying the speeds of blade coating, and/or utilizing multiple blade passes. Although, the doctor blade is a higher shear application method (in comparison to allowing the composition to flow naturally over a surface) there is no indication that their disclosure coupled a high shear coating method and apparatus with an appropriate shear-thinning composition. In fact, sections which describe the movement of a doctor blade and the velocities of the liquid composition at a distance from the blade indicate that shear-thinning compositions are not anticipated. Similarly, Japanese Patent Application Publications Kokai 61-114817 and 61-114818 (Morihara and Abe) from Fujutsu also indicate that higher viscosity compositions may be utilized with a doctor blade type apparatus, but make no mention of shear-thinning compositions and afford no indication of thickness of layers produced, amount of layer uniformity achieved, or the range of viscosity used and recommended.
Japanese Patent Application Publications Kokai 61-114817 and 61-114818 (Morihara and Abe) from Fujutsu also describe apparatus wherein a predetermined amount of liquid photo-setting resin is deposited within a container through an elongated supply opening. In one case, the liquid is laid as a uniform coating within the container to form the layers, and in another case, the liquid is deposited in the container and smoothed out with a smoothing plate (doctor blade). In both these cases, it is important to deposit a predetermined amount of liquid in the container. Too much liquid will cause either thicker layers than desired, or in the case of the smoothing blade, an excess of resin in the container at the end of the blade stroke. Too little liquid will cause thinner layers than desired, or in the case of the smoothing blade, a non-uniformly coated layer due to voids. 3-D Systems, in their European Patent Application 0 361 847, also has higher vat walls to contain the composition. In their disclosure, the composition is applied in excess by dipping the object surface below the composition surface level, however, the excess composition is not first added to the surface of the vat region and then removed by the doctor blade. Apparatus for the invention described herein, however, allow for the deposition of excess photoformable composition which can be directly removed by the extrusion head supplying the composition, or a doctor blade spreading the composition, or by other means. This is possible since the coating and imaging is performed at the surface of the vat, not in a region contained by higher vat walls, and therefore the excess composition can be scraped from the vat surface, collected, and recycled for use during fabrication of the object.
There is a French Patent 2583333 by an inventor Cilas Alcatel in which a nozzle is utilized to deposit pattern-wise a controlled amount of thixotropic photoformable composition on a surface. The resin is exposed and hardened as it is deposited. For reasons similar to those described above, the apparatus described in the Alcatel patent must apply only a predetermined amount of composition from the nozzle. If more or less composition is applied in this case, there will be a severe loss of accuracy. In addition, the use of a thixotropic composition in the Alcatel patent is for substantially different reasons than reasons disclosed in the invention described herein. In the Alcatel process, the composition must be deposited and stay deposited in the pattern-wise applied location at least until it is photohardened. The thixotropic behavior of the Alcatel compositions ensures that the composition retains its placed position during this intermediate period. However, for the invention disclosed herein, the composition is not applied pattern-wise and the thixotropic composition is being utilized to reduce distortions and to provide support to previously photoformed layers during coating.
In a recently issued U.S. Patent by Scitex Corporation Ltd. Pat. No. 4,961,154, (Pomerantz et al.) there is disclosed an apparatus employing a method of supporting an object, being fabricated layer by layer, wherein a second non-photosensitive liquid is supplied to both support the object layers and control the depth of photo-solidification of the individual layers. This non-photosensitive liquid may be a heated wax which presumably cools to a solid after application. Such art is only slightly pertinent to an embodiment of the invention described herein, where a heat liquefiable photoformable composition is applied as a viscosity reduced liquid, then allowed to cool to form a high viscosity or solid photoformable layer.
Also, pertinent to the use of thermoplastic photoformable composition processes are U.S. Pat. Nos. 3,264,103 and 3,395,014, (Cohen and Webers) issued in 1966 and 1968 respectively. In these patents, processes are disclosed in which a thermoplastic photohardenable composition is applied, in thin layers as a solution, to a film and then allowed to dry. This coated film is then exposed with UV or visible light imagewise from the film side thereby cross-linking the exposed portions. Next, typically a porous substrate, such as for example a felt, is placed on the coating side and the materials are heated by various means causing the unexposed portions of the composition to soften while the exposed portions, with a higher melting temperature, do not soften. By applying a pressure or a pressure differential between the film side and the felt side, the unexposed composition is blotted up by the felt and a three-dimensional relief surface is created on the film side. This process is directed primarily toward the production of printing plates, however, relief maps and "3-dimensional replicas of a photohardened relief image can be formed by application of heat and a pressure gradient to successive sheets of thermoplastic films in contact with said photohardened relief image". The processes described however, do not teach of applying the thermoplastic composition by heated coating means and do not teach the removal of the unexposed portions of the composition after several layers have been successively laid down and imaged as is taught in the disclosed invention herein. In addition, such "3-dimensional relief replicas" are actually contoured surface replicas rather than three-dimensional objects as described in the instant invention.